I don't know why but something about your post makes me think this has nothing to do with you having babies but about being happy in general.
It sounds to me as if you're looking for something and so seeing happy parents with their children may be making you think that if you were to have a child, you would be happy as well? I might be completely off here but there's just something off in your post.
I think you're just viewing the surface of these happy parents. There are things that go on in people's lives and homes that you will know nothing about. Don't we all look happy in our pictures? I find that most people lie about their lives. I have found out secrets of people that you wouldn't even believe. So before you go comparing yourself or feeling envious of others, consider that it may all be a lie.
I think you need to really question why, at this point in your life, you want to have a child so badly. I have clinical depression and I know what a roller coaster ride it can be. I, like you, have also gone through this phase. I then later realized I wanted a baby only because I was lonely. I have a great family, a loving husband and generally a good life but there was something missing. I thought I needed baby. I would've had one too if hubby didn't stop me. During love making, I would actually wish for a baby. Looking back now, I'm so glad it didn't happen.
Sometimes we go through emotional ups and downs and we don't understand why. If you truly have a desire for a baby (for all the right reasons) then you should but not with your husband. It isn't fair to him. If you need this, leave him out of it. Be with someone who feels as you do. The man will only grow bitter and resentful of you and maybe even the child. You both knew the deal when you married each other, it isn't fair to break it now after all this time.
I am very sorry for your emotional torture but I think you need to think things through a lot more before making any final decisions. Give yourself time and question your feelings. I wish you nothing but the best.